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Moe

CRT screen wobbles when in DOS but not Win98

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Hey everyone,

 

I have one of those old power strips that you place under your monitors.  The power strip works BUT it is interfering with the CRT when I'm in DOS (has to do with DOS refresh rate).  When I use this power bar the image shacks and wobbles on screen.  If I use a modern power bar places a foot or two from the CRT, no wobble.  I've tried to different Carts, same results.

 

I know it's the power strip causing the issue, but I REALLY want to use it in my setup.

 

Does anyone have any recommendations on how I could remove the interference?  Re-wire the internals of the power bar? Line the inside of it with some kind of ferrous metal? Wear a tinfoil hat while holding a clothes hanger? 

 

Thanks

Moe

 

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Either move the source of interference away or make some sort of shield.

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Never seen that kind of power strip, but; if its a plastic case, one solution is indeed to glue a layer of tinfoil inside to make a crude Faraday cage. which you'll ground to the power strip ground.

Else, slide a metal sheet between the power strip and the display.

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Thanks @CatPix,

 

Yeah, I love this power strip.  I had one on my childhood computer. I'll be disappointed if I can't keep it under my monitor. I didn't provide a good first picture, but you can see what they look like from these new images.

 

I opened up the power strip, not much to it.  I'm going to try and wrapping the exterior with tinfoil.  If that works I'll place tinfoil on the inside power strip. Though, I don't think a thin layer of tinfoil will be much help.

 

I don't have a lot of experience with power interference.  Could I add ferrite ring to all the internal wires?  Would that help?

 

Moe

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Since the switches are labeled, I assume you always use the same plug for the monitor.  Have you tried the monitor in a different plug?

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Posted (edited)

You can also try a noise arresting clip on ferrite bead.

 

OR-- Get an honest-to-goodness ferrite ring, and figure-8 the power cable through it.

 

 

Large ferrite ring

https://www.amazon.com/85X55X20mm-Transformer-3-3X2-2X0-8-Isolator-Inductor/dp/B07PDMWN57/ref=sr_1_4?dchild=1&keywords=large+ferrite+ring&qid=1621937111&sr=8-4

 

clip-on ferrite inductors for cables:

https://www.amazon.com/Taigoehua-Suppressor-Diameter-40X24X16mm-Ferrite/dp/B07ZNL5CDH/ref=sr_1_5?dchild=1&keywords=large+ferrite+ring&qid=1621937178&sr=8-5

 

 

Edited by wierd_w

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It seems like it's acting like a weak degaussing coil. First try straightening the interior wires and moving them equal distances from each other.    

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Posted (edited)
On 5/24/2021 at 7:41 AM, CatPix said:

Never seen that kind of power strip, but; if its a plastic case, one solution is indeed to glue a layer of tinfoil inside to make a crude Faraday cage. which you'll ground to the power strip ground.

Else, slide a metal sheet between the power strip and the display.

Tell us if the cover is magnetic or not.

 

Not confident an aluminum foil sheet will work. But yes. Try a ferrous metal sheet. Especially if the top cover is plastic or non-ferrous. Use something that will shield against magnetic lines of force/flux. It may have to be slightly larger than the powerstrip itself. It may have to overhang and cover the back and sides - maybe. But it does not have to be inside the powerstrip. Ground it for bonus points.

 

Farting around with repositioning wires is going to be tedious. You will need to do something to hold them in place and keep the right spacing, if you can discover it. I'll buy you a beer if you're successful with this method. Shit. I'll buy you beer either way.

Edited by Keatah
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I suggested inside because it would be invisible. It may not block all interferences but could do enough in this case.

A Faraday cage doesn't need to be made of thick metal. You're right on the grounding part tho, I forgot about that.

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Yes it could be enough. I was thinking outside because it needn't be cut precisely and for safety. Don't want it falling onto live wiring and stuff.

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8 hours ago, Keatah said:

Farting around with repositioning wires is going to be tedious. You will need to do something to hold them in place and keep the right spacing, if you can discover it. I'll buy you a beer if you're successful with this method. Shit. I'll buy you beer either way.

 

Difficult and tedious only to those with a severe mental handicap. 

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Posted (edited)

You should learn to be civil and polite.

 

And don’t make the OP feel bad when he tries an unimplementable solution.

Edited by Keatah
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Posted (edited)

Clipping ferite beads (like I linked to) to all the wires inside would make them less resonant, and less likely to create a bath of EMI inside that enclosure.  Finishing with a big ferrite bead on the power cord leading to the monitor would further isolate it.

 

The mixed kit has the clip-ons in various sizes, ranging from normal power cable, to thin data cable.  I would use the thin data cable ones on those loose wires inside, and would 3D print some cable management clips, then use double-stick foam tape to hold the printed clips in place.

 

You would probably have to take very accurate measurements of both the enclosure's geometry, the wire lengths, AND the EM fields being produced, than feed that into some expensive simulation software to get the ideal wire positioning to get the enclosure to do your EM dapening for you-- and as Keetah points out, that is no bueno, as you would need to make special clips to hold the wires in EXACTLY the right orientations and positions. Instead, I would just use some clip-on inductors to change the wires' properties, so that they cannot form resonant patterns/act like weak capacitors.

 

As for my .02$...

 

If I were to hazard a guess, I would say those ceramic disc capacitors (Likely there to prevent current bounce when you turn a switch on/off, and thus mitigate damage to a connected device) are acting like tanks, and are semi-tuned to a subharmonic of the power line's 60hz signal.  Adding the ferite bead on the green ground line that they are attached to should make them drain more reliably, and thus prevent the resonant tank behavior. (They would then do double duty as current debounce, AND as noise filter. The function as a tank is intended-- the resonant aspect is however, likely not. The added inductor will change the properties of the circuit, and is easily undone; Just unclip and remove the inductor if it seems to cause issues.)

 

Edited by wierd_w
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1 hour ago, wierd_w said:

If I were to hazard a guess, I would say those ceramic disc capacitors (Likely there to prevent current bounce when you turn a switch on/off, and thus mitigate damage to a connected device) are acting like tanks, and are semi-tuned to a subharmonic of the power line's 60hz signal.  Adding the ferite bead on the green ground line that they are attached to should make them drain more reliably, and thus prevent the resonant tank behavior. (They would then do double duty as current debounce, AND as noise filter. The function as a tank is intended-- the resonant aspect is however, likely not. The added inductor will change the properties of the circuit, and is easily undone; Just unclip and remove the inductor if it seems to cause issues.)

If you are talking about the blue capacitor-like devices that are poorly attached to the ground, neutral and hot wires, these appear to be varistors, for surge protection.

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Posted (edited)

There appears to be a brown disc capacitor in there also, along with an in-line diode rectifier.

 

image.thumb.png.f2dbb0f04e3b13dd4e46671f48c9df74.png

 

I can't really be sure though; Cannot read the reverse side of the brown disc, and the image is too blurry to make out the "diode?".

Edited by wierd_w

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It is 'dead bug' soldering, is what it is. 

 

Still, it looks like it contains a tank circuit, along with the surge supressing MOVs. The tank circuit is again, probably to handle the bounce when somebody kicks a switch on/off, so it does not spike the other devices in the parallel array. If I were looking for something that could introduce line noise in some khz-range frequency, it would be such a tank. (DOS vga modes are not exactly high horizontal refresh rate.)

 

Putting a clip-on inductor (that is easily removed) is easily tested, and if it improves the situation, great, and if not, the chokes are useful for other purposes.

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Hey everyone, thanks for all the good feedback.  If you guys want, I can take close-up pictures of the MOVs other items, if requested.

 

  •  The top and bottom of the case is magnetic.
  • I order some of those clip-on ferrite inductors and will test them on all the lines.
  • I will 3D print some cable spacers and try to keep all the wires equidistant from each other.

 

I'll post an update once I have the clip-on ferrite inductors and printed some spacers.

 

 

 

 

Cheers,

Moe

 

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Just for fun, you can try moving the black wire going to the master switch towards the right and hold it to the bottom with a piece of tape.  Then move the white neutral wire slightly to the right to give it a bit of space from the black wire going to the computer switch.

 

I've had power cords wadded up behind monitors that caused jitters or convergence issues and simply moving them cured the problem...no need to start doing equations for optimal distances.

 

 

 

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I suggest researching how high voltage lines are run in vacuum tube amplifier chassis to negate interference in the sound.

 

I would desolder the bus strip from the hot leg of the switches and run twisted pairs of hot and neutral lines to each switch. Run them around the bottom perimeter of the enclosure with three pairs on each side. Gut some old ATX power cords if you need the appropriate gauge wire. 16 AWG would be preferred but 18 AWG would be just fine for computer use.

 

Have you verified the wall outlet you are plugging this into is properly earth grounded? You can find testers for this at most hardware stores for about $10.

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I'm dubious of the ferrite solution.  Typically, its purpose is to suppress the mains "hum" on signal lines.  This time you WANT the mains.  Further, the mains are a far larger voltage than any potential signal interference would be.  Even further, this signal is converted to completely different forms inside all of the components connected.  Finally, the ferrite rings will not suppress any magnetic flux, so if the problem is really the mains inducing power into the monitor where it doesn't belong, I don't see how this will help.

 

The Faraday cage was a promising idea, but if the case is metal and grounded, it ought to suppress as much of the flux as a sheet of metal would.

 

The reason I asked about the plug is that the plug itself could have a bad contact or two, which could potentially under-power the monitor.

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10 minutes ago, ChildOfCv said:

The Faraday cage was a promising idea, but if the case is metal and grounded, it ought to suppress as much of the flux as a sheet of metal would.

Yes. Is the top of the metal case actually grounded it's screwed together? No paint acting as an insulator? No problem with the rubber insulating the screw heads?

 

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15 hours ago, wierd_w said:

There appears to be a brown disc capacitor in there also, along with an in-line diode rectifier.

 

image.thumb.png.f2dbb0f04e3b13dd4e46671f48c9df74.png

 

I can't really be sure though; Cannot read the reverse side of the brown disc, and the image is too blurry to make out the "diode?".

Looks like A2 AUPO which is a thermal fuse.

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